Damping means for mechanical vibratory devices



July 6, 1948. A. L. w. WILLIAMS ETAL 2,444,620

DAMPING MEANS FOR MECHANICAL VIBRATORY DEVICES I Filed June 23, 1944 2Sheets-Sheet 1 W ME WM mm INVENTOR 0mm (/MJm/Y ATTO EY.

DAMPING MEANS FOR MECHANICAL VIBRATORY DEVICES Filed June 25, 1944 July6, 1948. A. L. w. WlLLlAM E'l'AL 2 Sheets-Sheet 2 no mm :I

. INVENTOR. ALFRED L.W. WILLIAMS DEAN R.CHR|sT|AN BY fid- A O RNEYPatented July 6, 1948 DAMPIN G MEANS FOR MECHANICAL VIBRATORY DEVICESAlfred L. W. Williams, Cleveland Heights, and Dean R. Christian, Medina,Ohio, assigiiors to The Brush Development Company, Cleveland, Ohio, acorporation of Ohio Application June 23, 1944, Serial No. 541,694

9 Claims. 1

This application is a continuation-in-part of our application SerialNumber 429,896, filed February '7, 1942, and now abandoned.

This invention relates to mechanical vibratory devices in general and,more particularly, to damping means for such devices and for soundtransducers of the type wherein undesirable response peaks or resonancesoccur, such as microphones comprising piezoelectric crystal sections.

In order best to employ a piezoelectric crystal section, or an opposingpair of connected crystal sections such as are shown and described inRe. 20,213 and Re. 20,680, in a microphone it is expedient to transfersound-pressures thereto by way of an associated diaphragm havingappreciable stiffness. Such a diaphragm, however, together with thecrystal which it drives, has a definite resonance frequency that usuallylies within or just outside the range of sound-frequencies for which themicrophone is designed, giving rise to a highly objectionable peak inthe response curve of the microphone.

Attempts to eradicate the resonance peak, or peaks, have been made bydisposing damping material adjacent to a vibratile diaphragm, as shownin the patent to Stewart et al., 1,488,565, by disposing the diaphragmin a, housing open at the rear to the atmosphere through controllableorifices, and by numerous other means familiar to those skilled in theart.

Other vibratory devices, such as loud speakers, phonograph pickups,telephone receivers and the like, sometimes require damping; to them,also, this invention is broadly applicable.

Havin the foregoing requirements in mind, it is an object of ourinvention to, provide improved means for damping a vibratory mechanicalsystem.

An object of this invention is to provide a damping material capable "ofexerting high acoustic damping on a microphone diaphragm, or otherdiaphragm, adjacent to which it is disposed. I

Another object of this invention is to provide a damping material thatshall be so rigid as to be capable of being utilized in the constructionof a microphone housing.

Another object of this invention is to provide a damping material, ofthe type described, that 2 shall be unaffected by climatic conditionsand not subject to deterioration in ordinary use.

Another object of this invention is to provide what might be termed amicrophone cartridge wherein the casing thereof serves as damping meansfor the diaphragm or diaphragms therein that transfer sound pressures toa piezoelectric crystal element of the multi-plate flexing type.

Another object of this invention is to provide a unitary microphonehousing and damping element that shall be substantially unaffected byrough usage.

Another object of this invention is to provide a microphone wherein theusual housing or case is eliminated and one that may be manufactured ata relatively low cost as compared with heretofore known microphones.

Another object of this. invention is to provide a microphone that shallbe sealed against moisture and the operativeness of which shall not bedetrimentally aifected by changes in the static pressure of the mediumin which it is utilized.

Another object of this invention is to provide means whereby theextremely small motion of a vibrating piezoelectric crystal element mayeffectively be damped.

A still further object of this invention is to provide a dampingmaterial having such characteristics that it may itself, be utilized inthe construction of a microphone-housing or case.

In a preferred embodiment of this invention, the foregoing objects andother objects ancillary thereto are preferably attained by providing anovel damping material, constituted by a plurality of sintered-togetherminute discrete particles, which material is sound-permeable but issufficiently rigid to withstand reasonably rough treatment. Theparticles, preferably, are metallic spherules, and the material, aftersintering, is self-supporting and is suflioiently rigid to be utilizedas an element of the housing for a diaphragm-crystal assembly.Furthermore, the diaphragm is disposed very close to the inner surfaceof the housing-element, which surface has a contour complementary to thecontour of the diaphragm and the peripheries of the element and thediaphragm are sealed to provide a closed air chamber.

The novel features that are considered characteristic of this inventionare set forth with particularity in the appended claims. The inventionitself, however, both as to its organization and its method ofoperation, together with additional objects and advantages thereof, willbest be understood from the following description when read inconnection with the accompanying drawings in which:

Figure 1 is an exploded view in perspective of a piezoelectricmicrophone cartridge of the soundpressure type, exemplifying a preferredembodiment of the invention;

Figure 2 is an enlarged cross-sectional view of the assembled microphonecartridge;

Figure 3 is an enlarged sectional view of the microphone cartridge shownin the preceding figures, slightly modified for use at varyingaltitudes;

Figure 4 is a reproduction of a photomicrograph taken of a polished andetched portion of a damping plate section, the magnification being about'70 diameters;

Figure 5 is a plan view of an alternative embodiment of the invention,partly broken away to better show the interior construction thereof,

Figure 6 is a view, in vertical section, taken along a linecorresponding to the line VI-VI in Figure 5.

Figure 7 is a cross-sectional view of a modified form of microphoneembodying our invention takenv along line I'I of Figure 8, and

Figure 8 is a plan view looking toward the back of the microphoneillustrated in Figure '7.

In all figures of the drawing, identical elements are similarlydesignated.

Referring. now to the drawing, a preferred embodiment of this inventionis exemplified by a microphone comprisin a pair of similar quasiconicalmetallic diaphragms indicated generally by numerals I, I, disposed baseto base and enclosed in a housing constituted by two plates 3, 3 of theimproved sound-permeable damping material. 'It will be noted fromFigures 1, 2 and 3 of the drawing that each diaphragm has a centralportion 5,in the'form, generally, of a truncated cone, the truncatedportion of which is slightly dished, and that the base of the conicalportion of each diaphragm is surrounded by an annular ridge"! that istriangular in radial cross-section, the ridge, or corrugation, beingintegral with a fiat annular mounting rim 9.

Disp0sed between the diaphragms is a multiplate crystal element II ofthe twister type, one .pair of the diagonally opposite corners thereofbeing aifixed to the base of the conical portion of one diaphragm andthe other pair of diagonally opposite corners thereof being afiixed tothe base of the conical portion of the opposite diaphragm, Because ofthe configuration of the diaphragms, the central conical portion of eachmoves as a piston when the assembled device is subjected to soundpressures. use, the dia phragmsmove toward and from each other inunison, subjecting the crystal element to bending stresses arounddiagonals thereof, thus inducing potentials, atthe electrodes,corresponding to the combined excursions of the diaphragms, The crystalelement is provided with a pair of output leads I3, I3, connected to theelectrodes (not shown).

.In'Figure 1 the crystal element is shown merely as being affixed to oneof the diaphragms, but it is to be clearly understood that in theprocess of assembly the pair of opposite corners referred to are.cemented or otherwise afiixed to the op- .or the like, between whichextend, in the finished device, the conductive leads I3, I3 from thecrystal. element,

are disposed, respectively, on each side of the The cover or dampingplates 3, 3,

dual diaphragm crystal assembly and are clamped thereto by anencircling, internally channeled split ring I9 of Bakelite, hard rubber,metal, or the like, to form the diaphragm housing. The clamping ring I9carries two eyeleted terminals 2|, 2| through which the leads I3, I3extend, respectively, in the assembled device, and to which the saidleads are externally soldered.

The inner surface of each cover plate is spaced approximately 0.02 inchapart from the diaphragm by annular spacer rings 23, 23, which rings, inFigure 2 of the drawings, are shown as encircling and sealing theperipheries of the damping plates 3, 3. The rings, preferably, are madefrom a material such as Vinylite, that prevents vibrations from beingtransferred be tween the diaphragms and the damping plates and alsodamps vibration of the plates 3, 3 themselves.

The clamping ring I9, when sprung into place and fastened by anysuitable means (not shown) exerts radial and axial stresses on thediaphragm damping-element assembly, so compressing the spacing rings23,23 carried by the peripheries of the damping elements as toeffectively seal the rim ofthe assembly against ingress and egress ofair. Accordingly, during utilization of the microphone, any air flowbetween the interior and theexterior of the housing, such as would becaused by extraordinary excursions of the diaphragms at resonance, mustbe through the minute pores of the damping material, and such flow iseffectively impeded.

.The space between the diaphragms being sealed, as described, thetrapped air changes in volume as the'atmospheric pressure changes. Aminute opening 25, shown in Figure 3, may be provided through eachdiaphragm but in such case the crystal 'must be protected againstmoisture. For that purpose, a thin, flexible diaphragm 21, of rubber orthe like, is interposed between each of the sound receiving diaphragmsI, I and the crystal, as shown also in Figure 3. When such sealingdiaphragms are utilized, the crystal element is protected againstmoisture. Furthermore, changes in the volume of the trapped air areermitted by the flexible diaphragms, thus keeping stressesfrom beingimposed on the connections between the tabs I5 and the main diaphragmsThe material preferably utilized according to this invention for thecombined microphone housing and damping element is metallic incharacter. In the proper selection of this mate rial resides one of themost important phases of this invention. Specifically, the material inquestion is made by molding and thereafter sintering pedance.

a mass of spheroidal metallic particles in an inert or reducingatmosphere, at a temperature below the melting point thereof, to form ahomogeneous body or plate penetrated by microscopically small tortuousair passages that are irregular in extent and diameter. The metallicparticles are preferably of copper, globular or spheroidal in shape and,in the specific material utilized for microphone-damping plate of thetype described, they have an average diameter of the order of .004 inch.That is to say, the particles are such as will pass through a 100 meshscreen and be retained on a 150 mesh screen. The actual sizes of theparticles are evident from an inspection of Figure 4, which shows asmall section of a plate of the material, perpendicular to a facethereof, enlarged '70 diameters.

Although copper particles alone may be molded under pressure andsintered to form a plate of damping material, it is found moreadvantageous to utilize a mixture of 90% copper particles and tinparticles. The tin particles, which preferably are so small as to bepractically dust, wet the copper spheroids in the sintering process, andprovide better adhesion between them. Alloying of the copper particlesmay also take place, the final material then comprising a variety ofbronze.

After sintering, approximately 40% by volume of the material isconstituted by air passages and air pockets, designated by numeral 29 inFigure 4. The porosity of the material is dependent upon the averagesize of the spheroidal particles, the molding pressure and the sinteringtemperature; the finer the particles, the smaller the interstices and,consequently, the higher the acoustic im- The molding pressure must notbe so excessive as to materially deform the particles and to close thepassages between them. From an inspection of Figure 4 it will be evidentthat the particles 39 show little deformation which is a desirablecondition.

It is purely fortuitous that the particle-size appears variable inFigure 4. Such appearance is because the particles lay at randompositions with respect to the plane of the section examinedmicroscopically.

The sintering temperature should be below the melting point of the alloythat might be formed by the particles and the metal. If tin and copperare employed, sintering may be accomplished at temperatures ranging from1100 to 1300 Fahrenheit, depending upon the desired dampingcharacteristics.

Particles of other metals, such as nickel, for example, or of alloys,may be sintered together, or they may be mixed with still other alloyingmetals having lower melting points than the particles themselves andsintered. Instead of simply mixing the copper or other particles withparticles of tin, lead or analogous material, fair results may beobtained by coating the individual copper spherules with tin beforemolding and sintering. The method of coating copper particles with tinis well known in the art, and is described in U. S. Patent to George E.Best, 2,255,204.

In order to reduce resonance of the damping plates 3, 3 themselves, asmall percentage of lead particles or particles of an analogousnon-elastic substance may be mixed with the copper particles beforemolding and sintering. Furthermore, in lieu of a single plate, two ormore plates may be cemented together, at a number of spaced apartpoints, by Viscoloid or other non-drying adhesive. Preferably, ifseveral plates are cemented together, their thicknesses should bedifferent in order to further reduce resonance of the composite plate.Such modifications, it is believed, are sufficiently clear as to requireno illustration.

The damping plates 3, 3 are permeable to sound, and, although they areclosely coupled to the diaphragms, in use they oifer low impedance tonormal movements of the diaphragms compared to the stiffness of themechanical vibratory system including the diaphragms themselves. Atresonance, however, when th impedance of the mechanical system drops toa low value and the excursions of the diaphragms thereof tend to becomeexcessive, the rapid flow of displaced air through the plates isresisted by the minuteness of the passageways therethrough, thusimposing heavy damping on the diaphragms.

In greater detail, the action of the damping plates may be described asfollows: the sintered metal plates permit passage of air through theplates but the passages are so small that the flow of air meetssubstantial resistance. The plates thus act as acoustic resistanceelements. Due to the small size of the air passages, the acousticinertance of the passages is negligible compared with the resistance.The damping plates are disposed in sealing engagement with theperipheries of the diaphragms so that there are formed between theplates and the diaphragms small cavities which act as acoustic couplingelements between the diaphragms and the acoustic resistances. Thus theacoustic resistances are coupled to the mechanical vibratory systemconsisting of the crystal element and the diaphragms. Due to theproportions of the elements this resistance introduced into themechanical system is small compared with the mechanioal impedance of thevibratory system at frequencies well below and well above resonance. Theacoustic resistance therefore has little effect on the behavior of themicrophone throughout the lower part of its useful frequency range. Nearresonance, however, the impedance of the vibratory system becomes verylow so that the acoustic resistance has relatively great effect. Thus inthe neighborhood of resonance the mechanical system cannot vibratefreely at large amplitude under the stimulus of a small driving force asit would without the presence of the damping plates because if it wereto do so it would force relatively large quantities of air back andforth through the acoustic resistance elements dissipating therein asubstantial amount of energy. In order that the acoustic resistance mayexert sufiioient control on the mechanical system it is necessary thatthe acoustic resistance element be closely coupled to the diaphragm. Ifthe volume of this chamber is increased too much, the acousticresistance is short circuited by the acoustic compliance of the cavitiesso that it cannot exert proper resistance control on the vibratorysystem. We have found that in the device shown in Figures 1, 2, and 3 a,spacing of about .020 inch may be employed.

The net result of the utilization of damping plates constructedaccording to our invention is the realization of a microphone theresponse of which is substantially flat over a desired range and thesensitivity of which is substantially as great as that of a similardevice which is devoid of means for damping the diaphragms.

As disclosed in the Stewart et al. Patent No. 1,488,565, damping meanshave heretofore been proposed for microphones. Insofar as it is known,however, no one previous to this invention has proposed the use ofdamping material having such physical characteristics as to permit itsutilization for the housing of a microphone, nor has it previously beenproposed to utilize sintered metallic particles for such purpose.

From the foregoing description, it is not to be inferred that thisinvention is limited to a microphone of the double diaphragm type. Onthe contrary, it is equally as well applicable to microphones, such asshown in the Stewart patent, having only a single diaphragm and to allother acoustical apparatus wherein damping through controlled air flowis helpful.

This invention is also directly applicable to microphones of the generaltype disclosed in the United States Patent to Sawyer 2,105,010 and theUnited States Patent to Williams 2,126,437. That is to say, instead ofdisposing the damping plate adjacent to a diaphragm that actuates apiezoelectric crystal element or is actuated thereby, it has been foundadvantageous in some instances to mount the said damping plate in closeproximity to the crystal element itself.

A microphone, referring now to Figs. 5 and 6 of the drawing, embodyingthis invention may com prise a mounting frame designated in its entiretyby the numeral 3 l, which frame may be square as shown, or which mayhave any other contour desirable. The frame is provided with an interiorledge 33 to the upper and lower surfaces of which are cemented orotherwise aflixed plates 35, 35 of the sintered damping materialhereinabove described. Each plate carries two thin spacers 31, 31 ofsuitable material such as Bakelite, or the like, these spacers beingdisposed diametrically opposite each other at the midpoints of oppositesides of the plate. The spacers may be cemented or otherwise afiixed tothe surface of each damping plate or the plates, as indicated by dottedlines in Figure 6, may each be provided with a small channel for thepurpose of receiving the spacers and of holding them in fixed position.A piezoelectric element 39 of the multiplate bender type is centrallysupported upon each pair of the spacers 31, the outer surface of eachelement be ing cemented to a flexible covering sheet 4| which extendsbeyond the crystal element and is cemented to the adjacent edge of thesurrounding frame.

The spacing between the inner surface of each piezoelectric element andthe associated damping plate is extremely small to avoid shortcircuiting of the high acoustic resistance required to damp the crystalelements which have a high mechanical impedance.

At frequencies well below resonance the impedance of the crystalelements is so high that the resistance provided by the damping plateshas negligible effect on the vibrations of the elements. Near resonance,however, the mechanical impedance of crystal elements becomes very lowso that the resistance provided by the damping plates has a controllinginfluence, preventing strong resonance peaks. The volume of the centralspace is such that but little stiifness reaction is introduced thereby,the resistive damping action of the plates being predominant. Inmicrophones of the usual type, the spacing between the damping platesmay be of the order of A3 of an inch.

In order to still further increase the useful range of the microphonethus far described, in some instances it is desirable to utilize crystalelements having differing natural frequencies.

In the event that crystal elements having different natural frequenciesare utilized, they will resonate at different times during normalutilization of the microphone provided, of course, that both resonantfrequencies are not simultaneously present in the sound received. Whenone alone tends to vibrate at its resonance frequency, it will tend todisplace more air than the other element which is vibrating with normalamplitude. The extra air displaced will tend to move through theinterstices in the adjacent partition, and the friction developed willdamp the vibration to a greater or less degree depending upon thecomposition of the damping material, the spacing between the crystalelement and the plate, and the stiffness of the central air chamber. Asabove pointed out, the microphone is so designed that the damping platesdo not materially affect the motion of the crystal element during thereception of sound at other frequencies.

It will be noted that in the improved microphone construction shown inFigures 5 and 6 the damping plates are so disposed that they damp theresonant vibrations of the crystal elements without intercepting thesound waves which actuate the elements.

In a modified microphone the sintered metal damping plates may bepositioned to form portions of the protective housing, as has beenpreviously described, and in this modified construction the soundwavespass through the sintered metal damping material.

Figures '7 and 8 illustrate a pressure gradient microphone embodying ourinvention. It is comprised of a Bakelite or other electricallynonconductive base or frame member 45 which has an opening 48 in theback face thereof through which sound waves are admitted and a rigidfront member 41 comprised of porous sintered metal connected to the backmember by means such as connecting bolts 48. The peripheral portion of adiaphragm is is sealingly clamped between the edges of the front andback members 41, 45 and the remainder of the diaphragm is positionedclose to but spaced apart from the porous sintered metal member 41 toestablish an enclosure 50 which is substantially air tight except forminute air channels which extend through the porous sintered metal. Amultiplate flexing piezoelectric crystal element 5i of the twister typeis mounted with two diagonally opposite corners 52, 53 in drivingrelationship with the diaphragm 49 and with its other two diagonallyopposite corners 54, E5 braced against the frame member 45. Mountingpads and adhesive may be used between the crystal corners 52, 53 and thediaphragm 49 and between the crystal corners 54, 55 and the frame member45. Leads 6!), 6! from the crystal element iii are connected as bysoldering to terminals 66, 61 which are mounted within the microphone bymeans of tubular rivets 62, 63 extending through the frame member 45 andto which are connected outside terminals 64, 65 for connecting themicrophone into an electrical circuit.

As has been explained in connection with the previous figures, thevolume of the enclosure 50 and the number and size of the minute airchannels through the porous sintered metal 49 are so related to eachother and to the vibratory system that the resonance frequencyvibrations of the vibratory system comprised of the diaphragm 49 and thecrystal element 5! are effectively damped.

This microphone acts as a pressure gradient microphone due to the factthat both sides of the diaphragm 49 are directly accessible to soundwaves, on one side through the porous sintered tioned the fact that thisimproved damping mal terial is vastly more effective than any' materialsuchfas felt, blotting paper, wool or the like or material such as isdisclosed in the aforementioned Stewart patent. It is not knowndefinitely why this improved damping material is so effective but it isthought that its remarkable damping action results from the microscopicsizes and irregular directions of the air passages therethrough.

In addition, through the utilization of an improved damping material, itis possible to dispense entirely with microphone housings of usual typesand, accordingly, this invention permits of material reduction inmanufacturing costs.

The material does not deteriorate in use as does felt or the like, andit is substantially unaffected by climatic conditions.

Additional modifications and embodiments of this invention will beobvious to those skilled in the art to which it pertains. Thisinvention, therefore, is not to be restricted except insofar as isnecessitated by the prior art and by the spirit of the appended claims.

We claim as our invention:

1. In combination, a vibratory mechanical system comprising a diaphragmhaving a non-planar surface configuration, the system having a resonancefrequency, a substantially rigid member spaced apart from but closelyadjacent to the face of said diaphragm and including at least a portionformed of sintered-together minute discrete particles having a pluralityof minute air channels therethrough, means sealing said rigid member tothe peripheral edge of said diaphragm to form an enclosure substantiallyair-tight except for said minute air channels, the volume of saidenclosure and the number and size of the minute air channels throughsaid portion formed of sintered particles being such that sufficientacoustic resistance load is imposed on the said vibratory system toeffect substantial damping of its resonace frequency vibrations, thesurface of said rigid member facing said diaphragm having aconfiguration closely approximating the surface configuration of saiddiaphragm whereby said resistance load is imposed substantiallyuniformly on said diaphragm.

2. The combination as set forth in claim 1, further characterized inthis: that said rigid member comprises at least part of a protectivehousing for said diaphragm.

3. In combination, a resonant vibratory system including a diaphragmhaving a peripheral edge portion, a substantially rigid support for saiddiaphragm comprising a body of sintered-together minute discreteparticles sealingly engaging the peripheral edge portion of saiddiaphragm with the remainder of the said diaphragm disposed closelyadjacent to but spaced apart from the support to form an enclosuresubstantially air-tight except for the minute air passages through thesaid sintered body, the number and size of the said minute air passagesand the volume of the said enclosure being so related to each other andto the vibratory system that sufficient acoustic resistance isintroduced into the vibratory system to effect substantial damping ofits resonance frequency vibrations.

4. The combination as set forth in claim 3, furthe'r characterized inthis: that said rigid-stipport comprises at least part of aprotectivehousing for said diaphragm. g

5. In combination, a vibratory mechanical system comprising a pair ofinterconnected acoustical diaphragms disposed in face-to-face spacedapart relationship and having a resonance frequency, a pair ofsubstantially rigid bodies each associated with one of said diaphragmsand each spaced apart from but closely adjacent to a face of thediaphragm with which it is associated and each of said rigid bodiesincluding at least a portion formed of sintered-together minute discreteparticles having a plurality of minute air channels therethrough, meanssealing each of said rigid bodies to the peripheral edge of itsrespective diaphragm to form two enclosures each of which issubstantially air tight except for the said minute air channels in thesintered bodies, the volume of said enclosures and the number and sizeof the minute air channels through said sintered bodies being such thatsufficient acoustic resistance load is imposed on the said acousticalsystem to effect substantial damping of its resonance frequencyvibrations.

6. The combination as set forth in claim 5, further characterized inthis: that each of said rigid bodies comprises at least part of aprotective housing for said diaphragm.

7. In combination, a microphone cartridge including a non-planarvibratory element having a resonance frequency and a protective housingtherefor comprising porous sintered-together minute discrete particlesclosely acoustically coupled to said vibrator element substantiallythroughout the entire vibratory area of said vibratory element to actalso as an acoustic resistance element for said vibratory element tosubstantially damp the resonance frequency vibrations thereof.

8. In a transducer, a resonant vibratory system including a diaphragmhaving a front and a back face and having a resonance frequency, asubstantially rigid member spaced apart from but closely adjacent to oneof the said faces of said diaphragm and including at least a portionformed of porous sintered-together minute discrete particles havin aplurality of minute air channels therethrough and sealingly engaging theperipheral edge of said diaphragm to form an enclosure substantially airtight except for said minute air channels, the volume of said enclosureand the number and size of said minute air channels through saidsintered portion being such that sufficient acoustic resistance load isimposed on the said diaphragm to effect substantial damping of itsresonance frequency vibrations, and frame means connected to saidsubstantially rigid member and only partially covering the other face ofsaid diaphragm whereby sound vibrations may reach the said diaphragmfrom both front and back directions.

9. In an acoustic electro-mechanical transducer device, a vibratorysystem including a diaphragm having a peripheral edge portion, saidvibratory system having a resonance frequency within the frequency rangeof said transducer device, a plate-like body of sintered-together minutediscrete particles having a plurality of narrow tortuous pathstherethrough by which air can pass from one side of the plate to theother, sealing means connecting the peripheral edge portion of saiddiaphragm to the edge of said plate-like body with the face of saiddiaphragm closely adjacent to but spaced from a major face of saidplate-like body to form a cavity between the diaphragm and theplate-like body which is sealed from the outside exceptfor the saidplurality of narrow paths through saidplate-llke body,-the number andsize of the said narrow air paths-and the volume of the said cavitybeing so related to each other and to the vibratory system that-acousticresistance is introduced into the vibratory system to efiect substantialdamping of its resonance frequency vibrations.

ALFRED L. W. WILLIAMS. DEAN R. CHRISTIAN.

REFERENCES CITED The: following referencesare of record in the file ofthls'patent:

Number UNITED-STATES PATENTS Name Date Stewart et a1. Apr. 1, 1924-Wllliams a 1 Oct. 13, 1925 Nicholson Mar. 29,1932 Quinby 'June 28, 1932'Harshaw Jan. ,1, 1935 'Kuhn Nov. 3, 1936

